April 16, 2013

Solving an Evolutionary Puzzle:

Darwin’s Origin of Species outlines the foundation for the theory of evolution and described the idea that apes and humans had a common ancestor. It is not intuitive, however, to link species that look extremely different, such as fish and humans, on an evolutionary timeline.

According to Longo, as organs that are filled with gas, lungs and gas bladders are structurally similar. However, they have different functions. Lungs are vital organs for air-breathing vertebrates, as well as some fish and snails, which allow them to obtain oxygen from the air. Ray-finned fish, the largest class of bony fish, use gas bladders, also known as swim bladders, to control their depth in the water by inflating or deflating the sac with gas, usually oxygen. This organ allows fish to maintain a certain depth in the water without having to expend energy because of buoyancy.

According to an article in American Midland Naturalist by Prof. Alan Boyden, Rutgers University, analogous structures are examples of convergent evolution in which two similar anatomical structures developed independently but did not come from a common ancestor. For example, wings in bats and birds developed independently – the two did not have a direct common ancestor. Homologous structures are examples of divergent evolution in which two anatomical structures evolved from a common ancestor, Boyden said. Porcupine quills and seal fur are homologous structures because, although they now serve different purposes, they originally both developed from the fur of a common ancestor.

Because the structures of the two organs are similar, biologists have debated whether they are analogous or homologous structures, as well as what order they evolved in if the structures were homologous. While some researchers like Richard Owen and Charles Darwin believed that gas bladders evolved into lungs, others such as Prof. Karel Liem, Harvard University, believe that lungs evolved into gas bladders.

McCune, Longo and Riccio determined that lungs and gas bladders are homologous structures by showing that several ray-finned fishes, such as sturgeons and paddlefish, have pulmonary arteries similar to those used by organisms with lungs. Using computed tomography, they showed that the common ancestor of all ray-finned fish originally had lungs which were supplied by a pulmonary artery.

“Looking at the fish, you start to see that the gas bladder is serviced by the pulmonary artery primitively, and then the connection is lost when you get to the derived taxa,” McCune said.

To detect the similarities between the two structures, Longo looked at the one thing that all lungs share: their blood source. Pulmonary arteries service the lungs by providing the blood supply. Longo and McCune used a variety of techniques to analyze the fish, including dissection and corrosion casting. Corrosion casting is the process of injecting plastic into the vessel system of an organism and melting away the flesh to expose the structure that was created by the plastic. According to Longo, both techniques are destructive to experiment samples and do not allow for more detailed analysis of structures.

Computed tomography is an imaging procedure that uses computer-processed x-rays to create slices of specific areas of an organism. Digital processing is used to generate a 3-D image of the inside of an object from a large series of 2-D X-ray images.

“This is a 3-D matrix that you’re visualizing, and that is what allows you to look inside,” Riccio said.

Micro-CT scans enabled researchers to look at resolutions of 25 microns, about the width of a human hair.

“While the Micro-CT can only go up to 25 microns, it can accommodate living specimens,” Riccio said. Much like an MRI machines, CT-scanners rotate around the specimen and take multiple shots at different angles. “They allow us to visualize, quantify, and analyze to answer questions that no one had been able to answer before,” he said.

By studying the CT scans of the circulatory systems in lungfish, bowfin and several other related fish, the researchers were able to observe that other ray-finned fish have tiny vestigial pulmonary arteries which branch off from a parent vessel in the same way that pulmonary arteries branch off in organisms with lungs.

“It’s kind of like doing a digital dissection. The Micro-CT scans are like the Magic School Bus. With the CT scan, you can shrink and go on a field trip inside of the fish,” McCune said.

While the physical evidence provided by the fish answered whether gas bladders evolved from lungs, McCune’s lab also used genetic testing to cement its findings.

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